The Return of an Analog Synth Classic Johnston RI

In 1974, Tom Oberheim released the first Synthesizer Expander Module, or SEM. Players loved this easy-to-use, ballsysounding synth, and its unique multimode filter gave it a distinctive sound. The SEM went on to become the basis for some of the world’s first polyphonic synths, such as the Oberheim Two-, Four-, and Eight-Voice, which were essentially multiple SEMs in the same cabinet as a keyboard. Beginning in the late ’70s, these gave way to more compact analog polysynths (the Oberheim OB-Xa behind Paul Shaffer on page 28 among them) but analog tone nuts still hunt for original SEMs. Much to their delight, Tom has now reissued it.

HANDS-ON

1. You get 33 patch points on 1/8" mini jacks. The SEM is compatible with all one-volt-per-octave analog synths, transforming the SEM into a powerful synth expander module, hence the name!
2. Large coarse tuning knobs for each oscillator offer a five-octave range; small pots above fine-tune over a range of about a major third.
3. Unique multimode filter operates in lowpass and highpass modes and is continuously variable between modes with a knob — at 12 o’clock it’s a notch filter. Slide switch activates bandpass mode.
4. These knobs combine the oscillator waveform and mixer functions found in separate sections on other synths: Center is off, left makes the sawtooth louder, right does the same for the pulse wave.
5. Slide switch bypasses the VCA. Translation: infinite sustain. This also lets users run external audio inputs through the filter without triggering the envelopes — handy.

THE SIMILARITY STARTS HERE

In contrast to most second comings of great analog synth names, the SEM is almost identical to the original. In fact, the external cosmetics are changed far more than the innards. Tom tells us that just one component was changed from the original design due to lack of availability, and it doesn’t affect the sound. He kept the wedge-shaped beige case, and even the knobs are the same parts as on the originals. Some details have changed: two separate tuning knobs work better than the original’s touchy concentric arrangement. The biggest difference is the 33-jack patch panel. Since “bringing out” patch points is a common mod on originals, Oberheim took it one more step — a large step — providing fantastic flexibility for interfacing with other analog gear. Modular and Moogerfooger maniacs rejoice!

MIDI AND THE SEM The SEM’s extensive control voltage I/O is a boon for serious fans of analog, but what if you want to just wail on it from your MIDI keyboard? Tom Oberheim will soon release the SEM MIDI edition. This replaces the SEM’s left-side patch panel with a nicely outfitted MIDI-to-CV converter, and adds rear-panel MIDI jacks. Along with standard note on/off reception, there’s an auxiliary CV that you can control with your choice of velocity, mod wheel, or other continuous controllers. The aux CV destinations include oscillator frequen...

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